Time slot management method and a main station and substation realizing such a method

ABSTRACT

A time slot management method for use in a time division multiple access network. The time division multiple access network includes a main station (MS) coupled to each of a plurality of substations (S 1 , S 2 , S 3 , . . . , S 15 , S 16 ) via the cascade connection of a common transmission link (Lc) and an individual user link (L 1 , L 2 , L 3 , . . . , L 15 , L 16 ). The management method includes broadcasting substation identities (TEA 1 , TEA 12 , TEA 16 , TEA 3 , TEA 7 , . . . ) in downstream information packets from the main station (MS) to the plurality of substations (S 1 , S 2 , S 3 , . . . , S 15 , S 16 ) to allow each one of the plurality of substations (S 1 , S 2 , S 3 , . . . , S 15 , S 16 ) upon detection of its own substation identity to transfer a predetermined amount of upstream information packets in predetermined upstream timeslots to the main station (MS). The time slot management method additionally includes embedding at least part of the substation identities (TEA 12 , TEA 3 , TEA 7 , . . . ) in a physical layer operation and maintenance part. The physical layer operation and maintenance part is a predefined part of one of the downstream information packets broadcasted from the main station (MS) to the plurality of substations (S 1 , S 2 , S 3 , . . . , S 15 , S 16 ) to transport operation and maintenance information packets related to operation and maintenance functions. Such a physical layer operation and maintenance part can be a physical layer operation and maintenance cell (PLOAM).

[0001] The present invention relates to a time slot management methodfor use in a time division multiple access network as described in thepreamble of claim 1 and a main station and a substation realizing such amethod as described in the preamble of claim 8 and claim 9,respectively.

[0002] Such a time slot management method, main station and substationare already known in the art, e.g. from the published European patentapplication with publication number 0 544 975 A1. Therein, a time slotmanagement system is described which includes a time division multipleaccess system with a main station connected to a plurality ofsubstations by means of a tree like network. Substation identities arebroadcasted in downstream information packets from the main station tothe substations in order to thereby allow each one of the substationsupon detection of its own substation identity to transfer upstreaminformation in upstream information packets in predefined upstreamtimeslots. This time slot management system allocates the time slots ina flexible and dynamically way. The needed bandwidth to transmitupstream information is requested by the substations to the main stationand the substations are informed by the main station of the allocatedtimeslots.

[0003] A feature of the described time slot management system is thatthe downstream information packets are including a predetermined numberof blocks each of which include a header part and an information part.In order to reduce the overhead of the downstream information packets afirst step is realized to distribute the stream of substation identitiesover the header parts of this predetermined number of blocks and asecond step is realized to performe only one error check for thispredetermined number of blocks and to distribute the error checkinformation bits over the header parts of the predetermined number ofblocks. In this way, the overhead of one information packet is a reducednumber of bits.

[0004] It has to be remarked that in order to keep a maximum transfercapacity between the main station and the substations it is desirable tokeep the number of bits of the overhead of the information pockets aslow as possible.

[0005] An object of the present invention is to provide a time slotmanagement method of the above known type but wherein the overhead ofone information packet is further reduced.

[0006] According to the invention, this object is achieved due to thefact that the time slot management method of the invention includesembedding at least part of the substation identities in a physical layeroperation and maintenance part. Such a physical layer operation andmaintenance part is a predefined part of one of the downstreaminformation packets which is anyway broadcasted from the main station tothe plurality of substations in order to transport operation andmaintenance information packets related to operation and maintenancefunctions.

[0007] Indeed, by embedding substation identities in unused fields ofphysical layer operation and maintenance parts, which are a predefinedpart of one of the downstream information packets and which arebroadcasted anyway on a predefined regular base the number of bits ofthe overhead of one information packet is a reduced number of bits. Thismethod is described in claim 1 and is realized by the main station andthe substation described in claim in 8 and claim 9, respectively.

[0008] It has to be remarked that operation and maintenance functionsfor e.g. an Asynchronous Transfer Mode layered model are described ine.g. the book Asynchronous Transfer Mode: Solution for Broadband ISDNwritten by Martin de Prycker, and more particular in Chapter 3:Description of ATM according to CCITT from page 97 to page 124,published in 1991 by Ellis Horwood Limited, ISBN 0-13-053513-3.

[0009] Also the scope of the Recommandation ITU-T I.610, March 1993,previously CCITT Recommandation described in paragraph 1.2, page 1thereof, is to identify the minimum set of functions required to operateand maintain the Physical Layer and Asyncronous Transfer Mode ATM Layeraspects of the Broadband Integrated Services Digital Network B-ISDN usernetwork interface. Five phases are considered in specifying theoperation and maintenance OAM functions of the Broadband-IntegratedServices Digital Network B-ISDN which are described in more detail onpage 1 and 2 of this ITU Recommandation and which are listed hereafter:

[0010] Performance monitoring;

[0011] Defect and failure detection;

[0012] System protection;

[0013] Failure or performance information;

[0014] Fault localization.

[0015] Common operation and maintenance OAM cell fields forpoint-to-point conncetions are described in paragraph 7, ATM Layer OAMCell Format, pages 17-18 of this ITU-T I.610 Recommandation and thespecific fields for each type of OAM cells are described in paragraph7.2 Specific Fields for Fault Management Cell, pages 18 to 21 of thisrecommendation. As it can be seen from the description of these fieldsthe OAM cell indeed includes unused Operation and Maintenance cellinformation fields. By inserting, following the invention, in a formatsimilar to such above format but for point-to-multipoint connections, atleast part of the substation identities in unused fields of theoperation and maintenance cells less substation identities i.e. lessbits are inserted in the overhead of the information packets.

[0016] An important advantage of the time slot management methodfollowing the invention is that in the event when all substationidentities are inserted in physical layer and operation and maintenanceparts the time slot management method is more flexible towards anincrease of the number of substations. Indeed, with an increase of thenumber of substations more substation identities are required wherebythe number of bits of a substation identity also increases. In the eventwhen e.g. substation identities are distributed over the overhead ofdownstream information packets, an increase of the word length of thesubstation identities has an impact on the overhead of the informationpackets and eventually on the structure of the downstream frame format.However, in the event when all substation identities are inserted inphysical layer and operation and maintenance parts, an increase of theword length of the substation identities has only an impact on theorganization of the fields of the physical layer and operation andmaintenance parts.

[0017] It has to be remarked that the downstream information packets andthe upstream information packets may be carried as e.g. a continuousstream of cells in a cell based format with a cell based physical layer.Physical layer operation and maintenance cells are used for theconveyance of the physical layer operation and maintenance informationand are inserted in the continuous stream of information packets with apredefined insertion rate. Embedding at least part of the substationidentities in such physical layer operation and maintenance cells,shortly PLOAM cells, is a possible implementation of the method of theinvention.

[0018] It has to be remarked that a substation identity is notnecessarily a manufacturing number assigned during manufacturingfollowing a predefined unique and programmed manufacturing serialnumber. It can also be a ranging grant which is used during a rangingprocess whereby this ranging grant is generated by a main station inorder to initiate the ranging process. Two conditions for a substationto react on such a ranging grant is that the substation has not yetreceived an identification number from the main station and that itsmanufacturing serial number matches a mask given by the central station.If both conditions are valid the substation is allowed to react on sucha ranging grant. A feature of the method according to the presentinvention is that a substation identity embedded in a physical layeroperation and maintenance cell is a ranging grant. This is described inclaim 2.

[0019] Once a substation has received an identification number from themain station and the ranging process is completed, the substation isallowed to react on a data grant from the main station which includeshis identification number and is allowed to send data to the mainstation. A time slot management method according to the inventionwherein the substation identity is a data grant is described in claim 3.

[0020] A time slot management method following the invention used in anoptical communication network is described in claim 4.

[0021] Yet an important advantage of the management method following theinvention becomes clear in the event when a downstream informationpacket is an asynchronous transfer mode cell i.e. an ATM cell and in theevent when all substation identities are embedded in physical layeroperation and maintenance cells and no total overhead is required.Indeed, for such a format the downstream frame format of the downstreaminformation packets lines up with the requirements of the InternationalTelecommunication Union ITU-T Recommendation I.432, March 1993,Integrated Services Digital Network ISDN User NetworkInterfaces/Broadband Integrated services Digital Network B-ISDN UserNetwork Interface-Physical Layer Specification. This standard describesmore in detail on page 8 the interface structure of the physical layerfor a cell based interface consisting of a continuous stream of ATMcells, each containing 53 octets and with a maximum spacing betweensuccessive physical layer cells of 26 ATM layer cells. Such a physicallayer cell can either be an idle cell or a physical layer operation andmaintenance cell depending on the operation and maintenancerequirements. A time slot management method following the inventionwhereof the downstream information packets are organized following anasynchronous transfer mode cell ATM is described in claim 5.

[0022] It should further be noticed that the term “including”, used inthe claims, should not be interpreted as being limitative to the meanslisted thereafter. Thus, the scope of the expression “a device includingmeans A and B” should not be limited to devices consisting only ofcomponents A and B. It means that with respect to the present invention,the only relevant components of the device are A and B.

[0023] Similarly, it is to be noted that the term “coupled”, also usedin the claims, should not be interpreted as being limitative to directconnections only. Thus, the scope of the expression “a device A coupledto a device B” should not be limited to devices or systems wherein anoutput of device A is directly connected to an input of device B. Itmeans that there exists a path between an output of A and an input of Bwhich may be a path including other devices or means.

[0024] The above and other objects and features of the invention willbecome more apparent and the invention itself will be best understood byreferring to the following description of an embodiment taken inconjunction with the accompanying drawings wherein:

[0025]FIG. 1 shows a block scheme of an embodiment of a time divisionmultiple access network wherein the method of the invention is used;

[0026]FIG. 2 shows a downstream frame format and an upstream frameformat used by the time division multiple access network of FIG. 1.

[0027] Referring to FIG. 1 a time slot management method used in a timedivision multiple access network will be described. First, the workingof the time division multiple access network will be explained by meansof a functional description of the blocks shown in FIG. 1. Based on thisdescription, implementation of the functional blocks in FIG. 1 will beobvious to a person skilled in the art and will therefor not bedescribed in detail. In addition, the principle working of the time slotmanagement method following the invention will be described in furtherdetail.

[0028] The time division multiple access network includes a main stationMS and a plurality of substations S1, S2, S3, . . . , S15, S16. The mainstation MS is coupled to each substation S1, S2, S3, . . . , S15, S16via the cascade connection of a common transmission link Lc and anindividual user link L1, L2, L3, . . . , L15, L16.

[0029] The time division multiple access network is an optical networktransporting asynchronous transfer mode ATM cells over optical fibersfrom the main station MS to the substations S1, S2, S3, . . . , S15,S16. The goal is to have a network which is transparent for ATM cellsand which has an optimized throughput with minimal constraints on theoptical components. Optical passive splitters which are not shown in thefigure in order not to overload the figure are used in order to reachthe substations S1, S2, S3, . . . , S15, S16.

[0030] The time division multiple access network broadcasts substationidentities TEA1, TEA12, TEA16, TEA3, TEA7, . . . in downstreaminformation packets form the mains station MS to the plurality ofsubstations S1, S2, S3, . . . , S15, S16. Upon detection of its ownidentity a substation is allowed to transfer a predetermined amount ofupstream information packets in predetermined upstream timeslots to themain station MS. For example: upon detection of substation S3 of its ownidentity TEA3, substation S3 is allowed to send to the main stationupstream information packets in predetermined timeslots.

[0031] The main station MS includes a packet formatting module PFM,inserting means INS and queuing means Q. The queuing means Q is coupledto the inserting means INS which is included following this embodimentin the packet formatting module PFM.

[0032] Each substation, whereof only substation S3 is shown in detail inorder not to overload the figure, includes detecting means DET.

[0033] The functions of each functional block of above will be describedin the following paragraphs.

[0034] The time slot management network allocates the time slots in aflexible and dynamically way. Indeed the upstream transfer capacity ofthe time division multiple access network is shared amongst thesubstations S1, S2, S3, . . . , S15, S16 based on their needed andrequested upstream bandwidth to transmit upstream information. Thisneeded upstream bandwidth is requested by the substations S1, S2, S3, .. . , S15, S16 to the main station MS. The requested bandwidth istranslated by the main station MS in a predetermined number of allocatedtimeslots. This is realized by creating according the requestedbandwidth of the substations S1, S2, S3, . . . , S15, S16 a stream oftransmit enable addresses which are called in this applicationsubstation identities TEA1, TEA12, TEA16, TEA3, TEA7, . . . . It has tobe remarked that the detailed working of this allocation is described inthe cited patent application but goes beyond the scope of thisinvention. The aim is the use of the stream of substation identitiesTEA1, TEA12, TEA16, TEA3, TEA7, . . . to inform the substations S1, S2,S3, . . . , S15, S16 of the allocated timeslots. Following thisembodiment the stream of substation identities TEA1, TEA12, TEA16, TEA3,TEA7, . . . is provided to the inserting means INS by the queuing meansQ.

[0035] Physical layer operation and maintenance cells, shortly PLOAMcells, are also provided to the inserting means INS. Besides anotherimportant function which is described in a following paragraph, theinserting means INS inserts the PLOAM cells in the downstreaminformation packets. This means that such a PLOAM cell is in fact apredefined part included in one of the downstream information packets.

[0036] The content and the functions of such a PLOAM cell is describedin the introductory part of this application. In order to support thedownstream capacity provided at the main station MS and transmitted fromthe main station MS to the substations S1, S2, S3, . . . , S15, S16 itis desirable to insert a minimum number of PLOAM cells. Indeed, in thisway a minimum overhead is added. However, in order to make the interfaceof the network of this embodiment compliant to the ITU-T RecommandationI.432 the maximum spacing between successive physical layer cells is 26ATM layer cells i.e. after 26 continuous ATM layer cells have beentransmitted from the main station MS to the substations S1, S2, S3, . .. , S15, S16 a physical layer cell is inserted by the packet formattingmodule PFM in the downstream information packets in order to adapt thetransfer capability to the interface rate.

[0037] Referring to FIG. 2 the downstream frame format and the upstreamframe format used by the time division multiple access network of FIG. 1is shown. As it can be seen on FIG. 2, after 26 ATM cells a PLOAM cellis inserted.

[0038] The time slot management method of the invention is introducedthereafter i.e. inserting at least part of the substation identitiesTEA1, TEA12, TEA16, TEA3, TEA7, . . . in the PLOAM cells. This functionis realized by the inserting means INS. In the described preferredembodiment all substation identities TEA1, TEA12, TEA16, TEA3, TEA7, . .. are inserted in the PLOAM cells. In this way the requirements of theITU-T Recommendation I.432 are still supported and the overhead of adownstream information packet becomes superfluous.

[0039] This preferred embodiment also ensures that the boundaries of thedownstream frame format are in accordance with the boundaries of theupstream format. In this way it is easier to calculate the delay rangingi.e. the time needed for an information packet to travel from the mainstation MS to a particular substation e.g. substation S3 and back to themain station MS. By this way a symmetrical interface is realized and nostuffing bytes have to be inserted in order to align the downstreambitrate with the upstream bitrate. The accordance of the boundaries canbe seen on FIG. 2.

[0040] It has to be remarked that by choosing the number of upstreamcells in an upstream frame format as a multiple of the number of bytesof a downstream cell which is 53 bytes for one ATM cell the boundariesof the frames are aligned and the lookout of the upstream and downstreamframe format can easily be determined. A direct relation between thenumber of downstream cells in a downstream frame format and the numberof overhead bytes of one upstream cell is established. This means thatfor any length of an overhead of an upstream cell the number ofdownstream cells is a whole number. The above assumptions and relationsare given by the following formulae's whereby:

[0041] d: number of downstream cells in a downstream frame format;

[0042] C_(d): number of bytes of a downstream cell;

[0043] H_(d): number of bytes of the overhead of a downstream cell;

[0044] u: number of upstream cells in an upstream frame format;

[0045] C_(u): number of bytes of an upstream cell;

[0046] H_(u): number of bytes of the overhead of an upstream cell;

[0047] m: multiple of number of upstream cells in an upstream frameformat to the number of bytes of a downstream cell which is 53 bytes;

[0048] Assumptions:

[0049] 1. Downstream cell format and upstream cell format are ATM cellswhereby

length ATM cell=53 bytes=C_(d)=C_(u)

[0050] 2. H_(d)=0

[0051] 3. u=m*number of bytes of downstream cell=m*53

[0052] Conclusions:

[0053] In the event when the number of downstream bytes of thedownstream frame equals the number of upstream bytes of an upstreamframe:

[0054] (H_(d)+C_(d))*d=(H_(u)+C_(u))*u

[0055] (0+53)*d=(H_(u)+53)*m*53

[0056] d=(H_(u)+53)*m

[0057] Since it is more simple to integrate the involved functionality'sin the design of an application specific integrated circuits when theframe formats are shorter it is preferred in this embodiment to equalizethe number of upstream cells to the number of bytes of a downstreamcell:

[0058] u=m*number of bytes of downstream cell=m*53 with

[0059] m=1

[0060] u=53

[0061] d=H_(u)+53

[0062] When an upstream overhead of 1 byte is needed the number of cellsof the downstream frame format equals 54 whereby all parameters of thedownstream and upstream frame format are defined. In this particularembodiment the number of bytes of the overhead of an upstream cell isindeed chosen to be three.

[0063] Since every 26 downstream ATM cells a physical layer cell has tobe inserted it can be calculated that 2 cells of the 54 downstream cellshave to be a PLOAM cell. FIG. 2 shows 54 ATM structured cells whereof 2PLOAM cells and 52 ATM information cells.

[0064] Since an upstream frame format includes 53 cells and since foreach upstream cell a substation identity e.g. TEA3 is required in orderto transfer an upstream information packet in an upstream timeslot i.e.such an upstream cell from a substation e.g. S3 to the main station MSit can also be calculated that in these two PLOAM cells 53 substationidentities TEA1, TEA12, TEA16, TEA3, TEA7, . . . have to be inserted.Following this embodiment 27 substation identities are inserted in thefirst PLOAM cell of a downstream frame format and 27 substationidentities whereof one idle substation identity are inserted in thesecond PLOAM cell of a downstream frame format. This is realized by theinserting means INS of the main station MS.

[0065] An example of the allocation of operation and maintenancefunctions is given on page 10 of the above mentioned RecommendationI.432. Since this is a recommendation for point to point connections anda recommendation for point to multipoint connections is not yetavailable but is expected to be processed in the future it is alsoexpected to have some reserved fields available to insert substationidentities.

[0066] Like already mentioned above a substation e.g. S3 has to detectits own identity in a received PLOAM cell in order to be allowed totransfer an upstream information packet. This is realized by thedetecting means DET. In order not to overload FIG. 1 only for substationS3 the detecting means are shown DET(TEA3).

[0067] Although the principle working of the invention has become clearby the above description of the functionality's of each functional blockincluded in the mains station MS and the substations S1, S2, S3, . . . ,S15, S16 the consecutive steps of the method of the invention will berepeated here shortly.

[0068] A stream of substation identities TEA1, TEA12, TEA16, TEA3, TEA7,. . . and consecutive PLOAM cells are provided to the inserting meansINS.

[0069] The inserting means INS inserts the first 27 substationidentities of this stream whereof the identity of substation S3 i.e.TEA3 at predefined fields in the first coming PLOAM cell. It has to beremarked that in FIG. 1 the PLOAM cell after insertion of the substationidentities is shown as PLOAM′. The PLOAM cell is packed by the packetformatting module PFM into the downstream frame format and distributedto the plurality of substations. Substation S3 receives the PLOAM cellwhich is provided to the detecting means DET(TEA3). The detecting meansDET(TEA3) of substation S3 detects its own identity TEA3 in the PLOAMcell and knows that it is allowed to transfer an upstream informationpacket in a predefined upstream timeslot.

[0070] It should be noted that although the above described network ofthe chosen embodiment is an asynchronous transfer mode ATM network theapplication of the present invention is not restricted to the field ofATM. Small modifications, evident to a person skilled in the art may beapplied to the above described embodiment to adapt it to be integratedin other time division multiple access networks wherein physical layeroperation and maintenance parts are predefined in downstream informationpackets.

[0071] While the principles of the invention have been described abovein connection with specific apparatus, it is to be clearly understoodthat this description is made only by way of example and not as alimitation on the scope of the invention, as defined in the appendedclaims.

1. A time slot management method for use in a time division multipleaccess network which includes a main station (MS) coupled to each of aplurality of substations (S1, S2, S3, . . . , S15, S16) via the cascadeconnection of a common transmission link (Lc) and an individual userlink (L1, L2, L3, . . . , L15, L16), said management method includingbroadcasting substation identities (TEA1, TEA12, TEA16, TEA3, TEA7, . .. ) in downstream information packets from said main station (MS) tosaid plurality of substations (S1, S2, S3, . . . , S15, S16) to alloweach one of said plurality of substations (S1, S2, S3, . . . , S15, S16)upon detection of its own substation identity to transfer apredetermined amount of upstream information packets in predeterminedupstream timeslots to said main station (MS), characterized in that saidtime slot management method additionally includes embedding at leastpart of said substation identities (TEA12, TEA3, TEA7, . . . ) in aphysical layer operation and maintenance part (PLOAM), said physicallayer operation and maintenance part being a predefined part of one ofsaid downstream information packets broadcasted from said main station(MS) to said plurality of substations (S1, S2, S3, . . . , S15, S16) totransport operation and maintenance information packets related tooperation and maintenance functions.
 2. The time slot management methodaccording to claim 1, characterized in that at least one of saidsubstation identities (TEA1, TEA12, TEA16, TEA3, TEA7, . . . ) which areembedded in said physical layer operation and maintenance part (PLOAM)is a ranging grant.
 3. The time slot management method according toclaim 1, characterized in that at least one of said substationidentities (TEA1 , TEA12, TEA16, TEA3, TEA7, . . . ) which are embeddedin said physical layer operation and maintenance parts (PLOAM) is a datagrant.
 4. The time slot management method according to claim 1,characterized in that said time slot management method is used in anoptical communication network.
 5. The time slot management methodaccording to claim 1, characterized in that at least one of saiddownstream information packets includes at least one asynchronoustransfer mode cell (ATM).
 6. A main station (MS) for inclusion in a timedivision multiple access network wherein said main station (MS) is to becoupled to each of a plurality of substations (S1, S2, S3, . . . , S15,S16) via the cascade connection of a common transmission link (Lc) andan individual user link (L1, L2, L3, . . . , L15, L16), said mainstation (MS) including a packet formatting module (PFM) to insertsubstation identities (TEA1, TEA12, TEA16, TEA3, TEA7, . . . ) indownstream information packets in order to broadcast said informationpackets from said main station (MS) to said plurality of substations(S1, S2, S3, . . . , S15, S16) to thereby allow each one of saidplurality of substations (S1, S2, S3, . . . , S15, S16) upon detectionof its own substation identity to transfer a predetermined amount ofupstream information packets in predetermined upstream timeslots to saidmain station (MS), characterized in that said main station (MS) includesinserting means (INS) to insert at least part of said substationidentities (TEA12, TEA3, TEA7, . . . ) in a physical layer operation andmaintenance cell (PLOAM), said physical layer operation and maintenancecell (PLOAM) being a predefined part included in one of said downstreaminformation packets broadcasted from said main station (MS) to saidplurality of substations (S1, S2, S3, . . . , S15, S16) to transportoperation and maintenance information packets related to operation andmaintenance functions.
 7. A substation (S3) for inclusion in a timedivision multiple access network, said time division multiple accessnetwork including a main station (MS) being coupled to each of aplurality of substations (S1, S2, S3, . . . , S15, S16) including saidsubstation (S3) via the cascade connection of a common transmission link(Lc) and an individual user link (L1, L2, L3, . . . , L15, L16),substation identities (TEA1, TEA12, TEA16, TEA3, TEA7, . . . ) of saidplurality of substations being broadcasted in downstream informationpackets from said main station (MS) to said substations (S1, . . . ,S16) in order to allow each one of said plurality of substations (S1,S2, S3, . . . , S15, S16) upon detection of its own substation identityto transfer a predetermined amount of upstream information packets inpredetermined upstream timeslots to said main station (MS),characterized in that said substation (S3) includes detecting means(DET) to detect in a physical layer operation and maintenance cell(PLOAM′) its own substation identity (TEA3) inserted by said mainstation (MS) in said physical layer operation and maintenance cell(PLOAM), said physical layer operation and maintenance cell (PLOAM)being a predefined part included in one of said downstream informationpackets broadcasted from said main station (MS) to said plurality ofsubstations (S1, S2, S3, . . . , S15, S16) to transport operation andmaintenance information packets related to operation and maintenancefunctions.